This is most likely because those users belong to a company that you are not currently an administrator for. This is often the case with freelancers who have their own companies. In some cases you can be given admin rights to their company (if they consent to this), otherwise we at CtrlPrint will need to add them for you. Feel free to contact us if you need assistance.
It is important that if you do not see a user, who already has a CtrlPrint account when you are trying to add them to a project, that you do not create an additional account for them. Users should have only one account.